1. Field of the Invention
The invention relates to a method for forming bumps and more particularly, to a method for forming bumps on under bump metallurgy by micro-bubble film.
2. Description of the Related Art
It is common that a chip is electrically connected to external circuitry by wire-bonding in the art. However, more room is required to accommodate the bonding wires and the working frequency of the chip is also limited. Therefore, to solve the above problems, the flip-chip bonding technology has been developed to replace the conventional wire-bonding technology.
The so-called flip-chip bonding technology is first to form under bump metallurgy (UBM) on a chip and metal bumps are then formed on the under bump metallurgy. The chip can be connected to a substrate by the metal bumps with a reflow process.
According to the above flip-chip bonding technology, the structure of a metal bump formed by the conventional technique is illustrated in FIG. 1. A bonding pad, such as an aluminum (Al) pad 22 is formed on the active surface 27 of a wafer 20. A passivation layer 23 acting as an isolation layer is formed to overlay the active surface 27 of the wafer 20 and expose the bonding pad 22. An under bump metallurgy 24 is formed on the bonding pad 22. A metal bump 21 is then formed on the under bump metallurgy 24.
Referring to FIG. 2, the above under bump metallurgy 24 typically includes three layers. The uppermost layer of the under bump metallurgy 24 is a wetting layer 33 made of such as copper. The function of the wetting layer 33 is to provide a good bonding with a metal bump 21 made of stannum (Sn). The lowermost layer of the under bump metallurgy 24 is an adhesive layer 31. The function of the adhesive layer 31 is to provide a firm bonding with the aluminum pad 22. The middle layer of the under bump metallurgy 24 is a barrier layer 32 made of such as nickel/vanadium (NiV). The function of the barrier layer 32 is to prevent the wetting layer 33 from wetting the adhesive layer 31.
Referring to FIGS. 3a to 3j, the conventional method for forming a metal bump is to form a bonding pad 22 on the active surface 27 of a wafer 20. A passivation layer 23 is then formed on the active surface 27 of the wafer 20 and exposes the bonding pad 22. Afterward, an under bump metallurgy 24 is formed on the passivation layer 23 to overlay the bonding pad 22 (see FIG. 3a). A positive-type photoresist layer 42 is formed on the under bump metallurgy 24 (see FIG. 3b). Subsequently, a photomask 40 is positioned above the bonding pad 22 and a selective exposure process is performed on the photoresist layer 42. As a result, the portion of the photoresist layer 42 under the photomask 40 is not exposed to an ultraviolet light due to the shielding of the photomask 40 and therefore remains unchanged. The portion of the photoresist layer 42 exposed from the photomask 40 chemically reacts with the ultraviolet light (see FIG. 3c). The wafer 20 is then placed in a developing machine to have the portion of the photoresist layer 42 chemically reacting with the ultraviolet light removed by the developer (see FIG. 3d). Afterward, an etching solution, such as copper etching solution and aluminum etching solution is used to etch out the portion of the under bump metallurgy 24 not covered by the photoresist layer 42 (see FIG. 3e). Subsequently, the photoresist layer 42 is removed (see FIG. 3f). After the photoresist layer 42 is removed, a negative-type photoresist layer 50 is formed on the active surface 27 of the wafer 20. Next, a photomask 60 is positioned above the bonding pad 22 and a selective exposure process is performed on the photoresist layer 50. Consequently the portion of the photoresist layer 50 under the photomask 60 is not exposed to a light due to the shielding of the photomask 60 while the portion of the photoresist layer 50 exposed from the photomask 60 carries out a chemical reaction with the light (see FIG. 3g). The portion of the photoresist layer 50 not exposed to the light is then washed out by a developer and the portion of the photoresist layer 50 chemically reacting with the light still remains on the wafer 20 (see FIG. 3h). Afterward, a layer of solder paste 70 is applied to the portion of the under bump metallurgy 24 exposed from the photoresist layer 50 and the wafer 20 is subjected to a reflow process to have the solder paste 70 preformed a solder ball (see FIG. 3i). A strong base solution is then used to remove the remaining photoresist layer 50. The wafer 20 is subjected to a reflow process again to have the solder paste 70 formed a spherical metal bump 21 (see FIG. 3j).
According to the above method of using a strong base solution to remove the remaining photoresist layer 50, the strong base solution not only is expensive but also corrodes the under bump metallurgy 24 and passivation layer 23.
Accordingly, there exists a need to provide a method for forming bumps to solve the above-mentioned problems.